Dosimetry of irregularly shaped radiation therapy fields. II. Isodose contours obtained utilizing a simulated human thorax.

Abstract

Asimple method has been described (1) to improve the homogeneity of dose across large, irregularly-shaped cobalt-60 teletherapy portals by reducing the penumbral width. This is done by decreasing the distance between the lead lung shields placed within the treatment field and the patient. The resulting electron contamination can be reduced substantially by introducing an electron “filter” (tin). These earlier data were obtained using a rectangular, homogeneous (water) phantom. To further define the inhomogeneity of dose within the mantle field, a phantom was constructed to simulate the human thorax, and further dosimetric data were derived with the inhomogeneous phantom in the beam. Methods and Materials An experimental set-up was adopted which would allow (a) the production of isodose curves rather than measurements of depth doses at isolated points only, and (b) the study of dose variations at any horizontal plane across the field. A phantom was made of Mix D (2, 3) formed in the shape and size of the human thorax. Lungs were simulated by inserting polystyrene foam of appropriate shape as the liquid mix was poured. No bone was incorporated into the phantom. The phantom was cut into equal halves along a midcoronal plane. The anterior half of the phantom was then applied tightly to the front of the water tank of the isodose plotter (Toshiba model MRA-101-3B). The lung shields were aligned 8 cm from the surface of the phantom (Fig. 1). The reference point chosen for dose computation was the center of the mid-medi-astinal field (center of plane C, Fig. 2) at “minimum depth” within the water tank. This depth, 13.2 cm, was equal to the thickness of the phantom at that point, plus the thickness of the water-tank wall and one half the diameter of the isodose probe capsule. Thermoluminescent dosimetry methods utilizing lithium fluoride were used to determine the per cent depth dose at this reference point (4). Capsules containing LiF were embedded 0.5 cm deep in the phantom and others placed at “minimum depth” in the water tank. The value obtained, 55 per cent, corresponds closely to the value from standard depth-dose tables for similar treatment conditions (100 cm SSD, 30 × 30 cm field, 13.2 cm depth). The isodose plotter probe was then placed at the reference point as defined above, and its calibration controls were adjusted so that the probe registered 55 per cent. (Correspondence of LiF depth-dose values with those obtained using ionization chambers has been confirmed by Hendee et al. (5).) All isodose plots were then made without changing these controls. In this way, direct intercomparison of the isodose curves was possible without further computation. Isodose plots were then made through the four planes, A, B, C, and D shown in Figure 2.